<html>
 <body>
  <a href="http://sudopedia.enjoysudoku.com/Bivalue_Universal_Grave.html">BUG+4</a>
  <p>
  	If the <b>candidates {0}</b> were removed from the highlighted <b>cells {1}</b> each unsolved cell would have two candidates and each candidate would occur exactly twice in each row, column and block.<br/>
  	A Sudoku showing this so-called BUG (Bivalue Universal Grave) has either zero or two solutions rendering the puzzle invalid.<br/>
    Extra candidates in one of the unsolved cells are needed to avoid this <a href="http://sudopedia.enjoysudoku.com/Deadly_Pattern.html">deadly pattern</a>. 
  </p> 
  <p>
  	The highlighted <b>{1}</b> are the only cells with extra candidates: <b>{0}</b>.<br/>
	To avoid the deadly pattern at least one of the two cells must contain the <b>value {3}</b>.<br/>
	The other of the two cells must be a <b>{4}</b> because this digit can't go anywhere else in the highlighted <b>{5}</b>.<br/> 
	The <b>candidates {6}</b> can therefore be removed from the <b>cells {1}</b>.
	</p>
 </body>
</html>